1. Field of the Invention
The invention relates generally to the field of investment casting and more particularly to a method of casting superalloys having core cavities therein. The method employs a novel ceramic core consisting essentially of densified silicon nitride which is readily leachable in molten sodium hydroxide.
2. Description of the Prior Art
Metal temperatures of gas turbine components are generally controlled by circulating cooling air through complex internal passages and cavities in the blades and vanes of the turbine. These turbine components are generally cast from cobalt base and nickel base superalloys by the shell investment molding process. Preformed ceramic cores are generally used within the shell mold to form the complex internal passages in these cast components.
In the investment casting process, pattern wax is injected into a die around a preformed ceramic core or cores to duplicate the metal part to be produced. The wax replica together with the sprew and attached gating are dipped into a silica, zircon or alumina slurry using colloidal silica, aqueous sodium silicate or ethyl silicate as the vehicle. Slurry dipped pieces are stucco coated with refractory grain by dusting, tumbling or immersion in a fluidized bed. In usual practice, a 1/8 inch to 1/4 inch thick shell mold is eventually built up by repeated dipping, dusting and drying. After the core-wax pattern shell mold assembly is air dried, the wax is removed by flash firing. The mold with the ceramic core or cores secured within is then transferred to a vacuum casting furnace and preheated to about 1800.degree. to 1950.degree. F. The nickel or cobalt base superalloy is then cast into the shell mold at a temperature of about 2600.degree. F. After solidification of the metal, the shell mold is removed usually by fracturing it. Castings are then sand or vapor blasted to remove any adhering shell mold material. The sprew and gating are cut from the useful portion of the casting and the casting is dipped in a leaching bath such as sodium hydroxide to remove the ceramic core material from the internal passages formed therein. The leaching operation normally takes place at 1000.degree. to 1100.degree. F. with up and down agitation for a period of about 2 to 6 hours. The leaching media must preferably remove the ceramic core quickly and completely without attacking the metal alloy casting.
Generally, most of the ceramic cores presently in use are formed of a blend of silica and zircon. Core shapes are slip cast using water or ethyl silicate or injection molded utilizing various resin systems and plasticizers suitable for the extrusion. The molded silica zircon cores are then fired to produce a densified core body. Other popular core materials presently in use are aluminum silicate and fused silica. Fused silica and products of the silicate systems of the type set forth above are leached in molten caustic or hydrofluoric acid.
Other attempts have been made to provide cores made from calcium oxide compositions which are leachable in water, see for example U.S. Pat. No. 3,576,653 and U.S. Pat. No. 3,643,728. These formulations are difficult to handle however because calcium oxide absorbs water readily in air to form calcium hydroxide a compound which tends to crumble easily. Special handling techniques, therefore, must be employed with the calcium oxide compositions.
Another die casting core formed of sand, coated on its outer portion with a high melting point inorganic sealing salt, is disclosed in U.S. Pat. No. 3,501,320. This formulation is said to be readily leachable in hot water.
In the popular silicate system presently in use, core breakage due to poor strength contributes significantly to the rejection rate in finished castings. The strength of these currently used core materials cannot generally be increased without concurrently reducing solubility to levels where core removal is slow or even incomplete. Core density is therefore kept as low as practicable in order to increase the leach reactant surface area and also to reduce shrinkage of the core during final firing.
In addition to the density-strength problem of the silica materials, fused silica compositions are thermally unstable over 2000.degree. F. Devitrification with the crystallization of the beta crystobalite occurs with volume expansion. Upon cooling from an overfired condition, the beta to alpha transition is accompanied by large volume changes which may fracture the core. Large concentrations of crystobalite increase the thermal expansion in the core to the point where, upon heating, incompatibility causes the core to push out through the shell mold. The presently-used silica core is at best a compromise material which can survive the casting process and be removed from a casting only with difficulty.
Casting defects directly associated with the core materials of the prior art include: inclusions left in the alloy wall from core particle pull out; entrapped core material after leaching due to locally insoluble masses within the core or poor access of leaching media; unfilled areas on the wall of the casting resulting from core shift, breakage or distortion at the time of pouring; shell mold rupture or cracking resulting from thermal expansion mismatch between shell and core; and porosity in the casting due to reaction of molten metal with the core material to produce gases or evolution of the trapped gases from cores upon impingement of molten metal.